1. Field of the Invention
The present invention relates to a method of controlling the deactivation of a denitrating catalyst resulting from an exhaust gas dust in a boiler, a furnace or the like which employs a fossil fuel such as heavy oil, pulverized coal, COM, CWM, etc.
2. Description of the Prior Art
As environmental pollution grows worse, boilers, furnaces and the like which employ fossil fuels such as coal and petroleum suffer from the imposition of particularly strict environmental regulations these days. In regard to fuels, particularly coal and petroleum, those which have a relatively high content of N matter or S matter are relatively low in cost and are therefore in general use. Under these circumstances, the discharge of nitrogen oxides has become a serious world-wide problem. A particularly serious problem is the effect of nitrogen oxides on acid rain and other similar undesirable phenomena.
Examples of measures designed to reduce the generation of nitrogen oxides NOx from fossil fuels include: (1) improvements in burning techniques, for example, low oxygen combustion, two-stage combustion, exhaust gas recirculation combustion, and low NOx burner, (2) selection of fuel types (selection of fuels having a low content of N matter); and (3) development of exhaust gas denitration techniques. Among these measures, (3) is considered to be the most practical approach.
Examples of exhaust gas denitration techniques include: the catalytic reduction method in which NOx is reduced into N.sub.2 at 300.degree. C. to 400.degree. C. by means of a reducing gas such as ammonia in the presence of a catalyst; the catalytic decomposition method in which NOx is decomposed at 700.degree. C. to 800.degree. C. in the presence of a catalyst; and the absorption method in which NOx is absorbed into active carbon. Among them the catalytic reduction method which consists of a relatively simple process and utilizes ammonia is widely used and is regarded as being the most reliable. This invention relates to this dry ammonia catalytic reduction method.
The principle of this method is that NOx is reduced into N.sub.2 and H.sub.2 O generally by adding NH.sub.3 to the exhaust gas (300.degree. C. to 400.degree. C.) from a boiler outlet and then by passing the resultant mixture through a catalyst layer (for example, V.sub.2 O.sub.2, Fe.sub.2 O.sub.3, WO.sub.3, etc.) in a reaction vessel. This process is simple and suitable for treating a large volume of exhaust gas. The reaction formulae of this process are shown as follows: EQU 4NO+4NH.sub.3 +O.sub.2 .fwdarw.N.sub.2 +6H.sub.2 O EQU 2NO.sub.2 +4NH.sub.3 +O.sub.2 .fwdarw., 3N.sub.2 +6H.sub.2 O
Another method is the non-catalytic reduction method which consists of injecting ammonia into a high temperature area of about 800.degree. C to 1100.degree. C and effecting denitration in the absence of a catalyst. However, this method is hardly even used.
This invention relates to the above-mentioned dry ammonia catalytic reduction method. The most serious problem of this method is deactivation of the catalyst employed, which causes a reduced denitration efficiency and thus leads to the need for an increased amount of ammonia to be added. However, increasing the amount of ammonia added leads to an increased amount of unreacted ammonia passing through the denitrize, and this unreacted ammonia reacts with the SO.sub.2 and SO.sub.3 present in a gas to produce NH.sub.4 HSO.sub.4 which has a low melting point of 147.degree. C. Adhesion of this low-melting point substance to the elements of a downstream air heater (AH) causes clogging of the elements and a rising draft, which may in turn result in an unexpected shut-down. In order to prevent such a problem, inspection and repair of the catalyst becomes very important.
Causes of catalyst deactivation may be considered as follows:
(1) alkali metals such as Na, K, and alkaline earth metals such as Ca, Mg, Ba react with SO.sub.3 and the like to produce sulfates, which enter the catalyst receptacle to cause clogging of the catalyst;
(2) the surface of the catalyst may become coated with ash (particularly, Si, Al, unburnt matter, etc.) that is present in an exhaust gas, resulting in a reduction in the surface area of the catalyst;
(3) the catalyst may be poisoned and deactivated by sulfur oxides such as SO.sub.2 ; and
(4) a decrease in the amount of catalyst compounds (wear of catalyst components by dust and eluation of catalyst components by water).
In order to solve these problems, the catalyst is water-washed to remove any adhering matter after a boiler shut-down. If the catalyst function can be restored without stopping the boiler operation, great financial advantage will be obtained. Under these circumstances, a method of adding an iron compound powder just before and after the position of a denitrizer by using a sootblower is employed. The iron compounds added include Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4, Fe(OH).sub.2, Fe(OH).sub.3, FeCO.sub.3, FeOOH, etc.
However, this method has the following problems:
(1) since ordinary iron compound powders have large particle diameters, their activity is low, and a small specific surface area requires the addition of a large amount of powder;
(2) the use of an iron compound powder having a small particle diameter (about 0.1 .mu.m) increases the cost considerably, and since the particles are small, they are readily blown off rearward by means of the gas stream or the pressure from the sootblower, and therefore the proportion of particles adhering to the catalyst inside the denitrizer is uneconomically small;
(3) a powder surface with sharp angles causes erosion of the catalyst under the pressure applied by the sootblower, resulting in accelerated deactivation; and
(4) since the position where an iron compound powder is added is just before or near the denitrizer and the temperature (300.degree. C. to 400.degree. C.) thereat is therefore lower than the temperature (600.degree. C. or higher) at which the iron compound gains activity, most of the iron compound which is charged in large amounts does not function as a catalyst, resulting in extensive waste.
On the other hand, if an iron compound powder is added to a gas atmosphere with a temperature of 600.degree. C. or higher, a large amount of iron compound may be deposited on the heating surfaces of various devices which are disposed on the downstream side, such as a superheater (SH), a reheater (RH), a feedwater heater or economizer (ECO), etc., resulting undesirably in a rise in the exhaust gas temperature and an increase in the draft in the furnace.
Although iron compounds are inexpensive, they are readily poisoned and deactivated by SOx, and therefore employment of an iron compound alone limits any possible extension of the life of the catalyst. For this reason, methods have heretofore been proposed wherein an oxide of a heavy metal such as Ti, V, W or the like is employed as an active ingredient as well as an iron compound and is injected into the denitrizer using an ammonia injection nozzle or the like. These oxides of heavy metals are added in the form of an aqueous solution of an ammonia compound.
These methods, however, suffer from the following disadvantages:
(1) Since the denitrizer and structures in its vicinity are generally formed from structural carbon steel SS and the temperature near the denitrizer is about 300.degree. C. to 400.degree. C., addition of the above-described oxidizing water-soluble substance causes corrosion of the steel material.
(2) Since the position where the ammonia compound aqueous solution is injected is ahead of the position of the denitrizer, the injected solution cannot effectively be dispersed into the exhaust gas. Therefore, if there are a plurality of catalyst layers, the ammonia compound solution cannot be uniformly attached thereto, i.e., an excessive amount of the solution may adhere to the first layer, or the catalyst may partially be coated with the injected solution in excessively large amounts due to the action of a gas drift. Accordingly, in order to obtain effective results it is necessary to charge a large amount of the ammonia compound aqueous solution, i.e., 500 to 600 ppm or more.
(3) Most alkali metals in coal, such as K, Na and Mg, adhere to the catalyst layer in the form of sulfates. Therefore, if an additive in the form of an aqueous solution is injected ahead of the position of the denitrizer, water and steam wet the catalyst layer together with such sulfates and dust, and this leads to an increase in the amount of alkali sulfates, which are even more soluble in water, resulting in an increase in the amount of substance poisoned.
(4) The temperature at the position where the oxide of a heavy metal is added is about 300.degree. C.. to 400.degree. C.., which is much lower than the temperature (about 600.degree. C.. to 700.degree. C..) at which the oxide gains activity. Accordingly, in order to obtain adequate activity a large amount of the oxide must be charged. However, the addition of a large amount of the above-described oxidizing substance increases the rate of oxidation, i.e., SO.sub.2 .fwdarw.SO.sub.3, so that SO.sub.3 increases by a large margin and corrosion due to H.sub.2 SO.sub.4 is accelerated.
Thus, the addition of a large amount of these heavy metal substances ahead of the position of the denitrizer involves many problems.
It has heretofore been considered that vanadium compounds act as a strong oxidizing catalyst, have a low melting point and produce low-melting compounds such as n.Na.sub.2 O.mV.sub.2 O.sub.5 to corrode tubes in boilers and the like, and therefore they have been excluded from the group of substances which may be employed as additives for the abovedescribed purposes. On the other hand, tungsten oxides are known as oxidizing catalysts having a high melting point which act so as to cover the low-melting property of vanadium. However, these compounds have not been positively added to fuel.
If these substances are added in excessive amount, the rate of oxidation, i.e., SO.sub.2 .fwdarw.SO.sub.3, increases, and this leads to corrosion of boilers, furnaces and the like and causes an increase in the amount of slag on the heating surfaces. Therefore, the effect and side effects of the addition of such substances are greatly affected by the particle diameter and amount of iron compound charged and those of the vanadium and tungsten compounds added thereto. Accordingly, it is very important to select optimal particle diameters and amounts of these substances.